Unit 6 - Regulation of CHO Metabolism

Question 1

what’s the difference between enzymes that operate far from equilibrium and near to equilibrium?

Answer 1

far: rate limiting, well suited as control points b/c modulation has significant effect on flux thru a pathway (large negative delta G)
- rate of catalysis limits flux thru that step in a pathway
near: so efficient that a reduction in activity has little/no effect on overall flux
- rate of catalysis is much faster than net flux thru that step

Question 2

what are the 3 enzymes that operate far from equilibrium during glycolysis?

Answer 2

hexokinase, phosphofructokinase, and pyruvate kinase

Question 3

why is bypass III (glucose-6-phosphatase) not a good point to regulate glycolysis?

Answer 3

G6P is required for other pathways

-G6P released during glycogen breakdown enters glycolysis after bypass III

Question 4

why is pyruvate kinase a good point of regulation?

Answer 4

it needs to be turned off during gluconeogenesis b/c [PEP] (substrate) will be elevated

Question 5

what happens to PFK when an allosteric activator VS inhibitor binds?

Answer 5

activator: PFK assumes conformation that places stabilizing positive charge (R162) near negatively charged substrate
- causes glycolysis

inhibitor: PFK assumes conformation that places destabilizing negative charge (E161) near negatively charged substrate
- causes gluconeogenesis

Question 6

what stimulates or inhibits PFK? its bypass (II) FBPase?

Answer 6

PFK:
+ AMP, ADP, F2,6BP
- ATP, cAMP (indirectly)

FBPase
- AMP, F2,6BP

Question 7

what is F-2,6-BP structure and regulation? when is it active or inactive?

Answer 7

fructose-2,6-bisphosphate has synthesis and breakdown controlled by carefully regulated bifunctional enzyme (BFE) via hormones

• dephosphorylated state: catalytic site in kinase domain (PFK-2) is active to allow glycolysis
• phosphorylated: catalytic state in phosphotase domain (FBPase-2) is active to allow gluconeogenesis

Question 8

what stimulates or inhibits pyruvate kinase? pyruvate carboxylase and PEPCK?

Answer 8

pyruvate kinase:
+ FBP
- ACoA, ATP, cAMP-dependant phosphorylation

pyruvate carboxylase and PEPCK
+ ACoA

Question 9

which enzymes does glucagon affect?

Answer 9

effects are due to cAMP

• decreases glycolytic enzymes (hexo/glucokinase, PFK)
• increases gluconeogenic enzymes (PEPCK, FBPase, G6Pase)

Question 10

which enzymes does insulin affect?

Answer 10

decreases PEPCK of glugoneogenesis only

Question 11

what is the enzyme regulated to make glycogen? to break down glycogen?

Answer 11

to make glycogen: glycogen synthase

to break glycogen: glycogen phosphorylase

Question 12

how is glycogen phosphorylase regulated? how does this also inhibit glycogen synthase?

Answer 12

increased cAMP activates PKA, which activates phosphorylase kinase (b –> a), which activates glycogen phopshorylase (b –> a)
-active PKA also inhibits glycogen synthase and phosphoprotein phosphatase-1

Question 13

how is glycogen synthase regulated? how does this also inhibit glycogen phosphorylase?

Answer 13

insulin activates phosphoprotein phosphatase-1, which activates glycogen synthase (b –> a) and reverses all the PKA changes (inactivates glycogen phosphorylase (a –> b), phosphorylase kinase)

Question 14

how do glucagon and epinephrine affect liver cells?

Answer 14

glucagon: binds to receptor to produce cAMP, stimulate glycogen breakdown, and release of glucose into bloodstream
epinephrine: binds to alpha and beta receptors that increase Ca++ and cAMP respectively, for glycogen degradation

Question 15

does muscle have glucagon receptors?

Answer 15

no, because they are not responsible for maintaining blood glucose

Question 16

how do epinephrine and insulin affect muscle cells?

Answer 16

insulin: increases glucose uptake in muscle (and adipocytes) by increasing glucose transporters at cell surface
- increases glycogen and triacylglycerol synthesis
epinephrine: binds to beta-adrenergic receptors to increase cAMP, glycogen degradation, and glycolysis (by increasing availability of G6P)

Question 17

does moving of GLUT4 receptors from vesicles to membrane happen in brain or liver?

Answer 17

no, only in muscle and adipocytes by insulin

Question 18

allosteric regulation of glycogen phosphorylase

Answer 18

• when blood glucose is low, GP is phosphorylated (active R conformation)
• -even if blood glucose doesn’t drop, you must activate phosphorylase if AMP levels rise
• when blood glucose (and ATP and G6P) is high, GP is dephosphorylated (inactive T conformation)

Question 19

how is pyruvate dehydrogenase complex regulated?

Answer 19

complex contains its own regulatory enzymes E4 and E5 (pyruvate dehydrogenase phosphatase/kinase)

• PDK shuts off E1 by phosphorylating it
• PDP turns on E1 by dephosphorylating it (part of fight-or-flight response)

Question 20

what activates and inhibits pyruvate dehydrogenase kinase/phosphatase

Answer 20

PDK:
+ NADH, ACoA
- pyruvate, ADP

PDP:
+ Ca

Question 21

what are the product inhibitions of the TCA?

Answer 21

citrate synthase: citrate
isocitrate dehydrogenase: NADH
a-ketoglutarate DH: NADH, SCoA

Question 22

what are the feedback inhibitions of the TCA?

Answer 22

citrate synthase: SCoA

Question 23

what are the allosteric activations/inhibitions of TCA?

Answer 23

isocitrate dehydrogenase
+ ADP
- ATP